Aperture mask treatment to prevent cathode poisoning



Q 1959 M. EQNORDBERG APERTURE MASK TREATMENT TO PREVENT CATHODEPOISONING Filed Feb. v 25, 1957 APERTURE MASK TREATMENT TO PREVENTCATHODE POISONING Martin E. Nordberg, Corning, N.Y., assignor to CorningGlass Works, Corning, N.Y., acorporation of New York ApplicationFebruary 25, 1957, Serial No. 642,073 15 Claims. (Cl. 313-85) 7 Thisinvention relates to an improved perforate mask for assembly in acathode ray tube of the type used in multicolor television receivers andto a cathode ray tube embodying such mask.

Certain types of multicolor television systems operate on the generalprinciple of emitting a stream of electrons from one or more electronguns mounted in the neck of a cathode ray tube and directing such streamUnited States Patent a plurality of electron beams passing through theapertures or perforations in the mask to the patternof phosphors on theface. of the tube. Multicolor television systems of this sort aredescribed in detail in the patent litice being used in cathode ray tubesfor absorption of electrons.

By way of illustration the present invention is described with referenceto the accompanying drawings in which,

Fig. l is a schematic representation, largely in section, of a cathoderay tube and,

Fig. 2 is an enlarged fragmentary view in section of a perforate mask inaccordance with the invention.

-The cathode ray tube, schematically represented in Fig. 1 anddesignated by the numeral 10, has an electron gun 12 mounted in the tubeneck 14 and a viewing screen '16 composed of a phosphor coating 18, ofsuitable composition and pattern formulticolor televisingjapplied to theinside surface of transparent face plate or panel 20 which forms theopposite, enlarged end of the tube. The phosphor coating is applied inany known manner in a dot, stripe or other type. of pattern suitable forthe purpose. Perforate glass mask 2 2 is mounted intermediate electrongun 12 and screen v16, by any suitable conventional means, notspecifically shown.

In Fig. 2 a fragment of mask 22 is shown greatly exaggerated to betterillustrate chemically perforated apertures 26 which are too small andtoo numerous to be satisfactorily shown in a full viewsuch as Fig. 1..In accordance with the exemplary mask specifications, referred toearlier, the diameter of apertures 26 is 10 mils and they are spaced 28mils apart center to center. For purposes of illustration mask 22 isshown fiat, and apertures 26, as shown, are formed normal to theelectron intercepting surface. As a practical matter mask 22 willfrequently have imparted to it a definite curvature and apertures 26 maybe formed at angles up to 25 or so from normal depending on the locationand geometry of the tube components. The angle of each aperture will besuch as to provide alignment of electron gun, apertures and screenelements for proper screen scanning.

dimensions being held to a tolerance of 10.5 mils. The

inherent difficulties. in producing and handling such a mask are quitereadily apparent. Furthermore it is frequently necessary to sag orotherwise form such a mask sequent handling. It has been recognized thata mask.

composed of ceramic material such as glass should be highly desirable,but, until discovery of the chemical method of machining glass describedin United States Patent No. 2,628,160 issued to S. D. Stookey, nosatisfactory method existed for producing such masks. It was naturallyassumed that, if a mask of glass or other ceramic could besatisfactorily produced, its use in a tube should present no particularproblem since glass is used in construction of the tube envelope itselfand other ceramics are widely used in electronic tubes.

Once glass masks were successfully made by the Stookey method, it wasquite unexpectedly found that tubes embodying such masks in theirassembly tended to fail- In accordance with my present invention anelectron absorbing, perforate mask, for assembly and use in a cathoderay tube, is produced by firing a chemically perforated glass mask inareducingatmosphere containing free hydrogen. The efiectiveness of thistreatment indicates that the poisoning agent released during electronbombardment of an untreated glass mask is oxygen, and that preliminaryfiring ofthe glass in the presence of hydrogen, that is in accordancewith the present invention, causes such oxygen to combine with thehydrogen or to otherwise be rendered unavailable for subsequent release.This theory is supported by tests in which gases evolved from variousglasses under electron bombardment have been collected and analyzed,such test glasses including both hydrogen-fired and untreated glasscapable of being chemically perforated.

The atmosphere in which the glass is fired prefe rably contains at least25% hydrogen. While smaller amounts may be effective the amount of timerequired is usually too long for practical purposes. Theelfectiveness ofthe hydrogen firing increases with temperature and it is thereforedesirable to employ the maximum firing temperature' possible withoutdistortion of the glass. This maximum temperature will depend in largemeasure on the viscosity characteristics of the glass and will generallybe on the order of C. below the glass softening point.

However higher temperatures .may be employed Where,

filed October 10, 1955, in the name of Stanley D. Stookey and assignedto the assignee of the present application.

By way of specifically illustrating the present invention and itseffectiveness, reference is made to a test conducted on glass testpieces approximately ofan inch square by 50 mils thick and prepared froma glass composed essentially of 79.5 %'SiO 10% Li O, K 0,

4% A1 0 and 1% ZnO and containing minor amounts of Ag, Au and Ce0 asphotosensitizing agents and Sb O as a firing agent. These glass testpieces were given the standard thermal treatment which precedes etchingof this of each type was then placed in a special electron bombardmentapparatus, designed to permit collection and analysis of gas evolved aswell as a record of cathode operation, and was subjected to electronbombardment under cathode ray tube operating conditions of 25 kilovolts.In the course of this test it was observed that, when a test piece whichhad not been fired in hydrogen was being bombarded, there was a steadyevolution of gas, including a substantial proportion of oxygen, and acorresponding steady diminishing of cathode effectiveness. In contrastto this, no appreciable evolution of oxygen was observed duringbombardment of a test piece previously fired in hydrogen, and operationof the cathode remained normal during such bombardment, thusillustrating the effectiveness of hydrogen treatment on this glass.

The commercial significance of these results was confirmed in asubsequent actual tube test. In this test a cathode ray tube, havingmounted therein a chemically perforated glass mask fired in a hydrogencontaining atmosphere, was still successfully operating at theconclusion of an arbitrary thousand-hour test period, whereas similartubes containing identical glass masks, except for the hydrogentreatment, fail at intervals from a bundred to five hundred hours withan average life around three hundred hours.

While the invention has been described essentially with relation to whatis currently known as a shadow mask, it will be appreciated that it isnot restricted to a mask having the specific size and pattern ofperforations in such mask, but is equally applicable to other types ofperforate masks as well regardless of perforation pattern 7 or size.

What is claimed is:

1. A mask adapted to absorb electrons in a cathode ray tube and having aplurality of perforations extending therethrough which transmitelectrons to form a corresponding plurality of electron beams, said maskconsisting of a thin'sheet of perforated glass having a surfaceincapable of evolving cathode poisoning gas upon being subjected toelectron bombardment.

2. In a cathode raytube assembly including an image reproducing screen,an electron emission means, and a perforate mask interposed between theemission means and screen to form a plurality of electron beams byselective electron absorption, a perforated glass mask having a surfaceincapable of evolving cathode gas upon being subjected to electronbombardment.

3. A method of treating a perforate glass mask for incorporation into acathode ray tube which includes firing such mask in a reducingatmosphere composed in part at least of hydrogen and forming a surfaceon said glass which is capable of absorbing electrons withoutappreciable evolution of cathode poisoning gas.

4. The method of claim 3 wherein the reducing atmosphere contains atleast 25% hydrogen.

5. In the production of a glass mask adapted to absorb electrons in acathode ray tube, the method which comprises treating the mask in ahydrogen-containing atmosphere at an elevated temperature and formingthereon a surface capable of absorbing electrons without appreciableevolution of cathode poisoning gas.

References Cited in the file of this patent UNITED STATES PATENTS2,279,168 Kalischer Apr. 7, 1942 2,347,982 Bachman May 2, 1944 2,392,099Pollard Ian. 1, 1946 2,777,084 Lafierty Jan. 8, 1957 2,805,360 McNaneySept. 3, 1957

